The present invention relates to the materials of an electrode used in resistance welding. Currently the working life of electrodes, particularly of those to be used in spot welding, is relatively short. We have now developed a new electrode material, i.e. a copper alloy containing, in addition to the usual ingredients, a minimum of 20 ppm tellurium or other chalcogene, such as selenium or sulphur, and the service life of the electrodes has essentially increased owing to this new electrode material.
Spot welding is the most important method of fusion in the sheet metal industry. Automobile industry, for example, uses about n.times.10.sup.7 spot welding electrodes yearly. The purpose of the electrode is to apply the pressure force and electric current required in the fusion, as well as to cool the spot after the current is switched off. Thus the electrode material must have good mechanical strength, also at elevated temperatures as well as both electric and thermal conductivity. In order to improve the resistance to corrosion in the products, the use of coated sheets, such as hot galvanized or electrogalvanized sheets, is strongly increasing, particularly in the automobile industry. Respectively the use of aluminium, for diminishing weight, also is increasing.
The resistance welding of uncoated sheets has, during the past decades, developed to a well controlled process. The use of new coated sheets, however, has brought along a number of problems. The service life of electrodes used in the resistance welding of coated sheets is only a fraction compared to those used in the welding of uncoated sheets, and the sticking of the electrode to the surface to be welded may detach the electrode from its holder. As a solution to the problem of wearing, there have been tried alloys that increase the softening temperature, such as CuCdl or CuAg6 (ISO 5182 A1/2 and ISO A4/3) as well as age-hardening alloys, such as for example CuCrl and CuCrlZr (ISO A2/1 and ISO A2/2). Any essential improvement as for the service life of electrodes has not, however, been achieved with these alloys. In corresponding fashion, there have also been used the alloy CuZr (RWMA Group A Class 1) and dispersion-hardened copper DSCu (RWMA Group C Class 20), which have satisfactory wearing and sticking properties.
The patent applications DE 3,443,131, JP 60,227,997 and JP 9,001,074 describe various ways of coating or surface treatment methods for electrodes, and the patent application DE 2,808,392 specifies a method for the special heat treatment of electrode material. According to the U.S. Pat. No. 4,327,272, the electrode tip can be provided with steel balls. As far as is known, none one of these electrode types are in production.
Solving of the wearing mechanism of a resistance welding electrode lays the basis for lengthening the service life of the electrode. While welding an uncoated sheet, the strength, hardness and high softening temperature of the electrode help avoid the deformation of the electrode, and ensure a lengthy service life. However, while welding coated steel sheets or aluminium sheets, even the improving of the above mentioned properties does not lead to a satisfactory result, because a certain factor, even more essential to the outcome than the deformation, is the interaction of the electrode material and the surface of the sheet to be welded.
While welding a zinc-coated sheet with a copper-based electrode, the zinc of the coating and the copper of the electrode together form at the boundary surface an CuZn alloy with low conductivity. The alloying makes the electrode stick onto the sheet and may even detach the electrode from its adapter. On the other hand, the breaking of the CuZn layer on the electrode side wears the electrode rapidly. Moreover, the detaching may take place locally and cause spot-like pits in the electrode surface. During welding, these pits are quickly filled with Cu, Zn and Fe oxides, which are not electroconductive. This causes the current to be distributed unevenly in the object to be welded, and consequently creates an indefinite, unacceptable welding spot (nugget).
As already was maintained above, the requirements as for the usability and service life for the material of resistance welding electrodes include, among others, good thermal and electric conductivity, good mechanical strength and high softening temperature, which are achieved by alloying, and by means of work hardening, or precipitation hardening or dispersion hardening. In addition to this, it is important that the electrodes do not easily stick or form pits.